A tap changer with a motor drive is known from DE 4214431 [U.S. Pat. No. 5,736,827]. It is mounted outside the housing of a power transformer. The motor drive consists of numerous individual parts, inter alia also numerous electronic components. Belonging thereto are, for example, a drive motor, setting transmitter, microcontroller and evaluating unit. In order to ensure reliable operation of these electronic components, each component needs a voltage supply. In the case of the motor drive the voltage supply is realized by a single cable that connects it with a nearby voltage source.
Apart from the motor drive, numerous other electronic components are found in modern power transformers. Amongst these are numerous safety devices such as, for example, Buchholz relays, temperature sensors, air dehumidifiers and gas-in-oil sensors. They also each need a voltage source for operation. These safety devices are similarly arranged in the vicinity of the transformer and are connected with safety devices by respective cables. Since, for example, the Buchholz relay is mounted in the upper part of the power transformer. cabling, which is placed outside the housing, to the voltage source is correspondingly lengthy.
This conventional form of voltage supply, namely by lengthy cables with physically separate voltage sources, of the electronic components of the power transformers has numerous disadvantages. Since power transformers are frequently in use for several decades the cables also have to ensure reliable functioning during this time. Since, however, the plastic casing is exposed to different weathering conditions (rain, ultraviolet radiation, etc.) cable cracking or even breakages can occur. Due to the comparatively low voltage level that auxiliary apparatus requires, a special line provided only for these applications is required. This is not only cost-intensive, but also connected with a large amount of effort, since power transformers are often in districts without a low-voltage supply. The multiplicity of electronic components require special expert knowledge for installation, since errors can quickly occur during wiring. The remote voltage sources additionally hamper this. Notwithstanding these disadvantages, a voltage supply of the electronic components is necessary.